Multistage system for radiant energy flux transformation comprising an array of slat-like reflectors

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No. 10/339,123 filed Jan. 9, 2003, now abandoned which is a continuation-in-part of application Ser. No. 10/026,121 Dec. 17, 2001, now U.S. Pat. No. 6,971,756.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for concentrating and transforming radiant energy with a multistage energy flux transformation system. In particular, this invention relates to linear focus solar energy concentrators.

2. Description of Prior Art

It is well known that cylindrical parabolic mirrors and Fresnel lenses are used to concentrate the solar radiation which intensity is otherwise fairly low at the ground level for its direct use. While parabolic mirrors are notably superior in concentration over the lenses, this prior art design concept has the limitation of requiring tight shape and alignment tolerances to keep the concentrated sunlight focused onto a narrow target area. On the other hand, there is a limitation of energy collection ability of one-stage energy concentrators related to finite angle (one half degree) the sun subtends. As a result, the concentrated beam projected on the target has poorly defined boundaries because they are formed by the rays mainly emanated from the edge portions of the solar disk. These outer rays also have a longer path length giving rise to a larger transversal spread of the focal line.

Various arrangements have been proposed in the past for improving the sunlight collection of linear focus devices by introducing secondary optics into the concentrated beam reflected from the primary parabolic mirror. One of the major problems of such past proposals is the inherent problem of partial shadowing the primary concentrator by the secondary and relative inaccessibility of the focal line which hampers the utility of the devices.

The known multi-reflection systems, such as those derived from Cassegrain telescope optics, have a further drawback that the entire flux reflected by the primary mirror is entirely redirected back by the secondary mirror resulting in a longer path of concentrated flux and decreased concentration efficiency.

None of these previous efforts provides the benefits attendant with the present invention. The present invention achieves its intended purposes, objects and advantages over the prior art devices through a new, useful and unobvious combination of component elements and operation, at a reasonable cost to manufacture, and by employing only readily available materials.

It is an object of this invention to provide an improved radiant energy flux transformation system which increases the concentration of incident flux impinging on the primary reflector structure without disposing the secondary collector in the path of incident flux.

Another object of this invention is to provide an improved radiant energy flux transformation system which provides improved focusing for off-axis rays with minimum reflections and minimizes energy losses.

It is yet another object of the present invention to provide a system for radiant energy flux transformation which is composed by relatively simple optical elements and which is of compact and sturdy construction.

A further object is to provide an efficient reflective energy collecting system capable of substantially uniformly distributing the concentrated flux over a receiver surface.

Other objects and advantages of this invention will be apparent to those skilled in the art from the following disclosure and appended claims.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, the prior art problems are solved by a multistage system for radiant energy flux transformation comprising a primary concentrating collector being a rear-focus reflector structure and an elongated secondary collector. The primary collector is formed by an array of slat-like reflective surfaces having longitudinal axes extending parallel to each other and reflecting the incident energy to a plurality of converging directions to form a common linear focal area. Each reflective surface is tilted away from the direction to the energy source at an angle preferably less than 45° so that the incident flux is reflected from it at an angle being greater than 45° and not greater than 90° to provide a rear disposition of the focal area formed by the primary collector. The secondary collector is disposed in energy receiving relation with at least one of reflective surfaces of the primary collector to intercept and redirect at least a part of radiant energy flux reflected from the primary collector so that the efficiency of desired flux transformation is increased.

According to one aspect of the invention, in a preferred embodiment, there is provided a multistage system for radiant energy flux transformation in which reflective surfaces of the primary collector are designed and positioned to minimize screening and shadowing on other reflective surfaces. The primary collector can also incorporate two or more symmetric segments facing toward each other.

According to another aspect of the invention, when it is applied to transforming and utilizing solar energy, the focal line of concentrated sun rays is situated below the primary reflector structure with the advantageous result that the secondary concentrating collector can be disposed in a close proximity to said focal line without shadowing the primary collector and without associated energy loss. The multistage system for radiant energy flux transformation can further incorporate a photovoltaic receiver.

According to yet another aspect of the invention there is provided a multistage system for radiant energy flux transformation in which reflective surfaces of the primary collector have concave profiles represented by simple or compound segments of parabolic or circular shape. The shape of at least one reflective surface of the primary collector is selected so as to result in the transversal spread of the corresponding radiant energy flux reflected by the surface and impinging upon the secondary collector being smaller than the energy receiving aperture of the surface.

According to a further aspect of the invention there is provided a multistage system for radiant energy flux transformation in which the energy secondary collector can be mechanically separated from the primary collector. Furthermore, one or more reflective surfaces of the primary collector can be disposed in any one of a translated, a reversed and/or a rotated orientation relative to the others having the same basic arrangement.

DRAWING FIGURES

<figref idref="DRAWINGS">FIG. 1</figref> is a perspective view of a multistage system for radiant energy flux transformation in accordance with a preferred embodiment of the present invention;

<figref idref="DRAWINGS">FIG. 2</figref> is a perspective schematic view of an embodiment of the invention further comprising a photovoltaic receiver and a heat sink;

<figref idref="DRAWINGS">FIG. 3</figref> is a schematic orthogonal view of the system shown in <figref idref="DRAWINGS">FIG. 2</figref>;

<figref idref="DRAWINGS">FIG. 4</figref> is a detailed view of a secondary collector employing a pair of reflectors;

<figref idref="DRAWINGS">FIG. 5</figref> is a schematic orthogonal view of a further embodiment of the invention employing a flux homogenizer as the secondary collector;

<figref idref="DRAWINGS">FIG. 6</figref> is a schematic orthogonal view of a yet further embodiment of the multistage radiant energy flux transformation system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of flux transformation systems selected for the purpose of illustrating the invention include a primary rear-focus concentrating flux collector and an elongated secondary flux collector.

<figref idref="DRAWINGS">FIG. 1</figref> shows a perspective schematic view of a system <b>12</b> for concentrating and transforming radiant energy flux according to a preferred embodiment. System <b>12</b> includes a primary concentrating collector <b>14</b> comprising an array of cylindrical elongated reflectors <b>16</b> with longitudinal axes generally aligned parallel to a reference line (not shown), and an elongated secondary concentrating collector <b>22</b> extending parallel to reflectors <b>16</b>. The array of reflectors <b>16</b> comprises two symmetric segments where reflectors <b>16</b> are spaced apart and positioned adjacent to each other.

Reflectors <b>16</b> are individually tilted and aligned in a stepped arrangement, so that primary collector <b>14</b> has a linear Venetian blind-like configuration with the front longitudinal edges of reflectors <b>16</b> facing the source of radiant energy and the rear longitudinal edges of reflectors <b>16</b> facing away from the source. Reflectors <b>16</b> have mirrored surfaces <b>18</b> having concave transversal profiles to provide the energy focusing capabilities for each reflector <b>16</b>. The concave profile of each surface <b>18</b> is individually selected so that the respective energy beam reflected by the mirrored surface converges in the proximity of a predefined common linear focal area of collector <b>14</b>. Furthermore, reflectors <b>16</b> are arranged relatively to each other so that the concentrated beam formed by each reflector is at least partially superimposed with the concentrated beam formed by an adjacent reflector at the common focal area.

In order to entirely utilize the radiant energy received by the primary collector aperture, adjacent reflectors <b>16</b> can be appropriately spaced relatively to each other so that all incident radiation is intercepted and no energy is lost. Additionally, reflectors <b>16</b> can be arranged one with regard to the adjacent one in such a manner that the energy portions reflected by one reflector are not intercepted by the adjacent reflector.

It is important, according to the invention, that each reflector <b>16</b> is tilted away from the direction to the energy source at an angle preferably less than 45°. It will be appreciated by those skilled in the art that, as a matter of geometry, the angles of incidence and, consequently, the angles of reflection of radiant energy impinging on surfaces <b>18</b> will be greater than 45° and not greater than 90° thus providing the rear disposition of the focal area formed by primary collector <b>14</b>. Furthermore, according to a preferred embodiment, reflectors <b>16</b> can be positioned at successively increasing distances from and at successively increasing angles to the plane of symmetry of primary collector.

Secondary collector <b>22</b> should be disposed in energy receiving relation with at least one of mirrored surfaces <b>18</b> of primary collector <b>14</b> and located relatively remote from surfaces <b>18</b>. According to a preferred embodiment, secondary collector <b>22</b> can be a line-focus energy collector of a known type. By way of example, as shown in <figref idref="DRAWINGS">FIG. 1</figref>, secondary collector <b>22</b> can include two symmetrically disposed, curved trough mirrors facing toward each other. Secondary collector <b>22</b> is disposed so that its entrance aperture is facing primary collector <b>14</b> for receiving radiant energy and its smaller exit aperture is facing away from primary collector <b>14</b>.

Reflectors <b>16</b> can easily be fabricated using a number of means and materials. For example, reflectors <b>16</b> can be made of metal through extrusion of a metal part, roll-forming, slip rolling from sheet material, pressing, moulding, machining, or electroforming, and then polished on the reflecting side to obtain the required specular reflectivity for surfaces <b>18</b>. In an alternative example, plastic compound materials can be used for fabricating elements <b>16</b> and a foil or non-metal aluminized or silvered film can be used as a reflective material for mirrored surfaces <b>18</b>. A thin, sheet metal material can be used with a reinforced backing to provide the longitudinal stiffness for the reflectors. The required reflectivity can be achieved by incorporating a layer of highly reflective aluminum or silver laminated by a protective transparent layer of scratch- and weather-resistant plastic or other material.

Secondary collector <b>22</b> can include a pair of planar or curved reflectors which can be fabricated using similar methods and materials as reflectors <b>16</b>. Alternatively, secondary collector can be a refractive optical concentrator or flux homogenizer made from a transparent material such as glass or PMMA acrylics and redirecting the radiation by means of refraction and/or total internal reflection (TIR).

Multistage system <b>12</b> for radiant energy flux transformation forming the object matter of this invention can be based on a primary concentrating collector <b>14</b> comprising a number of reflectors <b>16</b> having individual parabolic transversal profiles and dimensions to obtain improved concentration of radiant energy. In view of that the construction of parabolic profiles can be relatively difficult, we propose a slight modification of collector <b>14</b> employing circular profiles for reflectors <b>16</b> or profiles formed by simple or a combination of two or more planar segments. Moreover, according to a further modification, reflectors <b>16</b> can be constructed with identical circular shapes and dimensions thus greatly simplifying the manufacturing process and enabling batch fabrication. Furthermore, one or more reflectors <b>16</b> can be planar or have a profile represented by a combination or a set of conjugate straight segments approximating a curved shape.

Reflectors <b>16</b> and secondary collector <b>22</b> can be interconnected or mounted to a frame in any suitable manner. For example, a frame may be provided which comprises walls (not shown) of metal, plastic, wood or other material extending transversely of the reflective element longitudinal axes at the reflector ends to support both primary and secondary collectors. Suitable tubular frame members (also not shown) may interconnect the walls to form a rigid structure.

System <b>12</b> can further comprise a receiver for receiving and converting the concentrated energy flux to whatever useful type of energy. For example, as shown in <figref idref="DRAWINGS">FIG. 2</figref>, a narrow-strip photovoltaic panel <b>24</b> can be provided for converting solar energy to electricity. Panel <b>24</b> can further include a heat sink <b>17</b> for heat extraction. Panel <b>24</b> can also be disposed in thermal relation to secondary collector <b>22</b> for improved heat dissipation.

System <b>12</b> can further incorporate a tracking device operatively connected to the primary and secondary collectors to follow the movement of the source of radiant energy. The tracking device may include mechanical, hydraulic, electric and electronic components such as are well-known in the art. By way of example, if system <b>12</b> is used to concentrate and utilize solar energy, a one-axis tracker can be employed with orienting the longitudinal axes of primary and secondary collectors in South-North direction and East-West tracking the movement of the sun. In another example, the receiver can incorporate a thermal collector of the planar or tubular shape for collecting the heat from the radiant energy flux cooperatively concentrated by both primary collector <b>14</b> and secondary collector <b>22</b>.

In operation, when system <b>12</b> is used to collect and transform solar energy, incident radiant energy RE strikes mirrored surfaces <b>18</b> of primary collector <b>14</b>. Each surface <b>18</b> concentrates radiant energy RE by reflecting the energy into a convergent flux and directing the flux through the space between a pair of adjacent reflectors <b>16</b> towards the common focal area of collector <b>14</b>. Secondary collector <b>22</b> disposed in the proximity of the common focal area of collector <b>14</b> receives at least a portion of radiant energy RE at the entrance aperture and further redirects and concentrates the energy onto a smaller focal area located at the exit aperture. According to a preferred embodiment of the invention, primary collector <b>14</b> composed by an array of reflectors <b>16</b> concentrates the incident radiation and directs it toward secondary collector <b>22</b> by means of a single specular reflection.

<figref idref="DRAWINGS">FIG. 3</figref> more fully illustrates operation of the system shown in <figref idref="DRAWINGS">FIG. 2</figref> when it is applied to transforming and utilizing solar energy. Referring to <figref idref="DRAWINGS">FIG. 3</figref>, incident rays <b>31</b>, <b>32</b>, and <b>33</b> of sunlight RE strike surfaces <b>18</b> of reflectors <b>16</b> arranged so that these rays are reflected from surfaces <b>18</b> and focused directly on the target area of receiving panel <b>24</b> using a single reflection. Incident ray <b>30</b>, which can be an off-axis ray emanated by a peripheral zone of the solar disk and/or a ray impinging on an edge zone of surface <b>18</b>, is reflected from surface <b>18</b> to a proximity of the focal area of primary collector <b>14</b> where ray <b>30</b> is intercepted by secondary collector <b>22</b> and redirected to panel <b>24</b> so that no energy is lost and net concentration is improved. In other words, energy portions reflected from the respective surfaces <b>18</b> of primary collector <b>14</b> are individually focused onto a relatively small entrance aperture of secondary collector <b>22</b> and at least partially intercepted by the secondary collector. Secondary collector <b>22</b> further transforms the energy flux cooperatively formed by surfaces <b>18</b> at the entrance aperture so that the concentrated energy flux formed at the exit aperture and projected on target panel <b>24</b> will have a smaller transversal spread and sharply defined boundaries and the radiant energy will be further intensified.

In an embodiment, secondary collector <b>22</b> can be designed to intercept and redirect only peripheral parts of the concentrated flux formed by primary collector <b>14</b> without intercepting the central rays of the primary flux. This produces an improved concentration and flux uniformity using minimum reflections and minimum energy loss. Furthermore, since the individual concentrated beam formed by an uttermost reflector <b>16</b> will strike a planar target such as panel <b>24</b> at a greater incidence angle compared to the beams formed by inner reflectors <b>16</b>, it will tend to produce the largest focal spot on the target. Therefore, in an embodiment of the invention, secondary collector <b>22</b> can be designed to intercept the off-axis rays from only one or more outermost reflectors <b>16</b> and allow the rest of concentrated energy to pass through collector <b>22</b> without being redirected by collector <b>22</b>. This operation is illustrated in <figref idref="DRAWINGS">FIG. 4</figref> which also shows the irradiance distributions formed by primary collector in target plane <b>28</b> with and without secondary collector <b>22</b>.

Referring to <figref idref="DRAWINGS">FIG. 4</figref>, an incident ray <b>42</b> of sunlight RE reflected from one of the central reflectors <b>16</b> of primary collector <b>14</b> strikes target plane <b>28</b> at a point P₄without being intercepted by secondary collector <b>22</b>. At the same time, an extreme off-axis ray <b>41</b> reflected by an outermost reflector <b>16</b> strikes secondary collector <b>22</b> at a point P₁and is redirected by collector <b>22</b> to a point P₃of target plane <b>28</b> within the boundaries of the exit aperture of collector <b>22</b>. Obviously, the size of the focal spot of system <b>12</b> will be defined in this case by the size D₁of the exit aperture of secondary collector <b>22</b>, and the resulting irradiance distribution on the target plane <b>28</b> will have relatively sharp boundaries such as those described by a profile <b>45</b>.

Should there be no secondary collector <b>22</b>, ray <b>41</b> would strike target plane <b>28</b> at a point P₂which is located at a greater distance from the center of the focal zone of system <b>12</b> than point P₃. Thus, without secondary collector <b>22</b>, the energy distribution in the target plane will have a greater transversal spread D and highly non-uniform profile <b>44</b>. Since secondary collector <b>22</b> is designed to result in D₁being smaller than D, the concentration ratio being improved. Additionally, as illustrated by the comparison of profiles <b>44</b> and <b>45</b> in <figref idref="DRAWINGS">FIG. 4</figref>, the redistribution of the peripheral off-axis rays according to this invention improves the uniformity of the concentrated flux.

Other Embodiments

<figref idref="DRAWINGS">FIGS. 5 and 6</figref> show other embodiments of the invention.

When system <b>12</b> is used to collect and convert solar energy, secondary collector <b>22</b> can be any concentrating solar concentrator of a know type. However, this invention is not only limited to this, but can be applied to the case where collector <b>22</b> further transforms the concentrated beam formed by primary collector <b>14</b> without additional concentration. For example, a system can be provided which redistributes the concentrated flux in order to obtain a better uniformity on the target. <figref idref="DRAWINGS">FIG. 5</figref> shows a schematic orthogonal view of system <b>12</b> where secondary collector <b>22</b> comprises two reflective walls of planar shape to provide homogenization of the energy flux concentrated by primary collector <b>14</b>. This can be useful, for example, for improving the performance of panel <b>24</b>. The reflective walls should have a sufficient depth to allow for multiple reflections for at least a substantial portion of the concentrated flux striking the entrance aperture of collector <b>22</b>.

The foregoing embodiments are described upon the case when the array of elements <b>16</b> of primary collector <b>14</b> comprises two symmetric segments disposed at an angle to each other. However, this invention is not only limited to this, but can be applied to the case where only one segment is used (asymmetric design), for example, as illustrated in <figref idref="DRAWINGS">FIG. 6</figref>. Secondary collector can be a parabolic trough or planar rectangular mirror which can intercept at least a part of concentrated energy flux reflected from uttermost reflectors <b>16</b>, for example, to provide a desired flux convergence or normal energy flux incidence onto panel <b>24</b>.

Alternatively, reflectors <b>16</b> can be organized in two or more arrays that can be tilted, rotated, and positioned differently relatively to each other and secondary collector <b>22</b>. There are also various other possibilities with regard to the dimensions, number and relative disposition of reflectors <b>16</b>, as well as individual curvatures of surfaces <b>18</b>. In addition, one or more individual reflectors <b>16</b> can be selectively added, omitted, changed or replaced in primary collector <b>14</b> to provide a desired operation. Dimensions, curvatures and relative dispositions of reflectors <b>16</b> can be varied so that the concentrated beams reflected from respective surfaces <b>18</b> can be made partially overlapped, contacting, or spaced apart. It will be appreciated that primary collector <b>14</b> and secondary collector <b>22</b> can be designed in a large variety of ways so that the energy distribution in the focal line will be tailored to a desired irradiance profile.

The foregoing embodiments are also described upon the case when reflectors <b>16</b> have fixed positions relatively to each other. However, this invention is not only limited to this, but can be applied to the case where reflectors <b>16</b> can be rotated around their longitudinal axes and/or moved relatively to each other and secondary collector <b>22</b>. Alternatively, secondary collector <b>22</b> can be moved and/or rotated, for example, to intercept different portions of the concentrated energy flux reflected from reflectors <b>16</b> of primary collector <b>14</b>.

Although the above description contains many specificities, these should not be construed as limiting the scope of the invention but are merely providing illustrations of some of the presently preferred embodiments of this invention. While a variety of embodiments have been disclosed, it will be readily apparent to those skilled in the art that numerous modifications and variations not mentioned above can still be made without departing from the spirit and scope of the invention.